1. Field of the Invention
The present invention relates to a method for trimming a printed resistor and, more particularly, to a method for trimming a printed resistor formed on an insulating substrate in a hybrid integrated circuit (IC).
2. Description of the Related Art
FIGS. 4 through 9 show plan views of conventional printed resistors having various kinds of slit patterns. In each of these figures, a resistor 1 is formed extending over a pair of electrodes 2a and 2b provided on an insulating substrate 3 by means of screen printing or the like. Slits 41 through 46 are formed in the resistors 1 by trimming to adjust the resistance value of the resistor 1.
Among the slits 41 through 46 formed by trimming to adjust the resistance, the slit 41 shown in FIG. 4 is formed by trimming so as to extend from one edge of the resistor 1 in parallel with the electrode 2a and to be bent perpendicularly approximately in the shape of L.
The slit 42 shown in FIG. 5 is formed by trimming as a continuation of the slit 41 trimmed approximately in the shape of L so that the new slit returns toward one edge of the resistor 1 approximately in the shape of a square bottomed J.
The slit 43 shown in FIG. 6 is formed by trimming in the shape of J starting from one edge of the resistor 1.
The slit 44 shown in FIG. 7 is formed by scan-cutting off a portion of the resistor 1 from one edge of the resistor 1 between the electrodes 2a and 2b.
Further, the slit 45 shown in FIG. 8 is formed by trimming in the shape of U the tops of which extend from one edge of the resistor 1, the width of the U extending from the electrode 2a side to the electrode 2b side.
The slit 46 shown in FIG. 9 is formed by trimming (lean cutting) one end of the resistor 1 linearly between the electrode 2a and the electrode 2b while also cutting parts of the electrodes 2a and 2b.
The conventional trimming methods described above have had the following problems.
First, resistors having the L-shaped slit 41, the square bottomed J-shaped slit 42 and the J-shaped slit 43 as shown in FIGS. 4 through 6 are susceptible to changes in resistance value due to a surge. More specifically, as shown in FIG. 10(a), a current density is distributed non-uniformly in the printed resistor 1 having a L-shaped slit 41, so that a current is concentrated at points D and E which are located near the bending portion and an end portion of the L-shaped slit 41. As a result, microcracks occur at points D and E or the resistor burns at points D and E when the resistor is subjected to a surge. This causes the change of resistance of the resistor. For example, the resistance of these resistors shown in FIGS. 4 through 6 change with 3.350% on average before and after a surge in a lightning surge test.
Second, although the method of forming the slit 44 by scan-cut as shown in FIG. 7 brought about a good surge resistance and it can be described as an effective trimming method, it takes a considerable amount of time for the trimming, thus raising the cost of the product.
Third, while the method of forming the slit 45 by trimming approximately in the U-shape as shown in FIG. 8 is is done quickly while maintaining the surge resistance of the scan-cut shown in FIG. 8, there is a possibility that it turns out to be a J-shaped slit (similar to one shown in FIG. 6) as the trimming is terminated during the trimming of the U-shape due to a dispersion of an initial value of the resistor. As a result, there is a possibility that this resistor will suffer from the aforementioned problem.
Fourth, in the method of forming the slit 46 by a lean-cut shown in FIG. 9 (trimming the resistor 1 and the electrodes 2a and 2b), the trimming is quickly done while maintaining the surge resistance similar to the method of forming the slit 45 by trimming in the U-shape. However, it has been very difficult to program the necessary trimming machinery to completely cut both electrodes. The resistor and occasionally the electrodes have not been completely cut, resulting in a parallel electrical connection of the resistor and thus the method lacks reliability.
Accordingly, it is an object of the present invention to solve the aforementioned problems by providing a resistor trimming method which brings about a good surge resistance and which allows a slit to be formed in the resistor quickly and reliably. It is another object of the present invention to provide a resistor having a slit formed by the resistor trimming method of the present invention.